Human malignant and benign prostatic tissues are obtained from surgery and established in monolayer culture as well as in an animal model, the athymic nude mouse. Extracts are prepared from the surgical tissues and from the in vitro grown cells and are examined by competition radioimmunoassay for the presence of antigens which react with antisera prepared against antigenic determinants common to RNA tumor viruses. Extracts demonstrating viral antigenic activity are further analyzed by a three-phased, two-dimensional immunoelectrophoresis system designed to size and isolate the reacting proteins. Patient sera are also collected and analyzed for immunologic responses to prostatic tissue extracts in radioimmunoassays and for reactions with isolated interspecies viral antigens. Localization of viral-like proteins on the surfaces of prostate cells is determined by peroxidase and ferritin labeling utilizing the electron microscope. Pronase extracts are prepared from the tumor removed from athymic nude mice and prostate-specific proteins are identified by column chromatography, slab gel electrophoresis and isoelectric focusing. Heterologous rabbit antisera are then raised to these isolated prostate-specific proteins. The heterologous serum reactions are also compared to the patient serum reactions against the same proteins.